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Fetal Anatomy

December 14, 2007

The Baby's Position: Time For Delivery

The position of the unborn baby...where do they go and how do they get there?

The baby's head is the largest part of his or her body, so when a baby delivers head-first, it usually follows that...well... the rest of the baby follows. But there are many variations from the standard head-first descent of the presenting part, like breech, transverse, shoulder-first, etc. Even among the head-first deliveries, the face can be pointing down or up, or even face-first. It's these little surprises that keep an obstetrician or midwife on alert with each seemingly "routine" delivery.

In the head-first category, the shape of the mother's pelvis can determine which way the head is placed. The most common way is face-down (Occiput Anterior), and this allows the easiest measurement to clear the pelvis. This position is especially helpful because the head can pivot up against the pubic bone, allowing the chin's appearance to finish the delivery of the full head. Face-up, called Occipital Posterior (back of the head down), is a more difficult delivery because the baby can't flex the head on exit. It's like delivering a baby a pound bigger. It's not impossible, but it's more difficult to push with effectiveness, and the pushing stage of labor can last longer than usual. If the head is sideways, then the rotation necessary for delivery is incomplete. Called "Occiput Transverse," the head can usually be gently rotated by the obstetrician to face-down (Occipital Anterior) for a normal delivery.

Asynclitism is a word that describes a head-first position wherein the head is tilted to the right or left from the midline. Many people feel that an epidural given before the head has descended well into the pelvis causes the maternal muscles that normally guide the head down correctly to get relaxed, resulting in a sloppy, unguided descent. It's a hard positioning to work with and can lead to the need for C-section.

And then after all of the head-first positions, there's every other way.

Breech Position

Breech refers to feet- or buttocks-first presentation. This is a real thinking obstetrician's dilemma, because, as mentioned above, the largest part of the baby is the head. So delivery of the feet or buttocks creates a scenario wherein larger and larger parts of the baby have yet to prove the ability to clear the pelvis. In other words, if the head won't fit out in a head-first baby, delivery can be effected via second-choice¡Vthe C-section. But in a breech delivery, if the head won't fit out, the rest of the baby already has. The cord is out as well and is compressed in the birth canal on it's way up to the placenta. This is such an emergency with such a bad outcome that most obstetricians feel that breech = C-section.

But why do some babies come out head first ("vertex" presentation) and some breech?

Babies tend to seek the most comfortable position in their mothers' wombs (uteri). If the largest part of the baby is the head, then over time the baby will fidget and maneuver around until the head gravitates to the largest space in the uterus. That most generous space is the lower uterine space. And usually at 32 weeks this position will stick.

So a breech, by the laws of physics, will be in such a position if there's some problem with the lower uterus NOT being the biggest space. For instance, a low-lying placenta or placenta previa can occupy enough space so that the biggest space is up high. Also, congenital abnormalities can make other parts of the baby the biggest part, meaning THAT part will become the lowermost presenting part. In fact, in my training it was always a warning to check for abnormalities when there was a breech presentation.

In a "frank breech," the buttocks are first. In a "footling" breech, one or both feet are first the "single footling" breech and the "double footling" breech. The difference depends on whether the knees are bent or not. In both, the hips are flexed, but if the knees are straight, then the lower legs, along with the thighs, are bent over the baby's abdomen, resulting in the frank breech. If the knees are bent, then the feet are positioned back toward the cervix and the outside world.

Thankfully, most breeches are in that position for, officially, "unknown" reasons. Mostly normal breech babies are delivered by C-section, putting to rest the fear of congenital problems. Some brave obstetricians (I'm a coward) are doing vaginal deliveries of breech babies, but this isn't acceptable unless the baby is estimated to be at least a pound less than a mother's previous largest baby, the baby is a frank breech, and the maternal pelvic measurements are generous. Any breech that is a first baby should still be born by C-section, because it's easier to explain a possibly unnecessary C-section than to explain a baby that was traumatized by the too large "after-coming" head. There are many who warn that even successful vaginal deliveries of breech babies result in what are called "soft" neurological signs that is, not brain damage, but Attention Deficit Disorder, dyslexia, hyperactivity, and the like.

Occasionally I'll encounter a baby that's in a crazy position, like transverse (whole body sideways) or shoulder-first. C-section is the safest way to address this malpresentation; this is common sense when the unified vector forces of labor don't push a head-first baby outward, but instead crunch a baby that's not pointing straight down.

Delivering Twins

If you think the placenta can crowd out an adequate space, imagine what an extra baby does to the locale! Twins will compete for the most comfortable space, but usually there's a membrane which will separate them and favor one to be lower. The big risk here, though, is a breech baby that's first, compared to the head-first second twin. In this positioning, the head of baby 1 (the breech), may sit just above the head of baby 2 (the higher of the babies). When labor ensues for a vaginal delivery, it's possible to have the horror of interlocking heads. Needless to say, a breech/head-first presentation of twins necessitates a C-section.

But head-first/head-first twins can deliver vaginally, as can head-first/breech. Except, once again, when cowardice rears it's common-sense head. There can be a considerable wait sometimes for baby #2 to descend, and with this there's the possibility of a prolapsed cord which could cut off oxygen to this twin. Because of this, although not mandatory, some doctors do C-sections on all twins.

Delivery of a baby (or babies) requires a knowledge of the baby's position. A baby's position is sometimes for a reason, but often there's no explanation for alternatives to the most common head-first ("vertex"). With third and fourth generation antibiotics and advanced surgical technique, the risk to the mother from a C-section is now outweighed by the benefit to the baby when there's positioning that would make vaginal delivery hazardous. In contrast to the dangerous times of just a generation ago, we no longer need to seek heroic vaginal deliveries for these babies. And even using this way out, the C-section rate can still be kept low by waiting for proper descent of the baby's head into the maternal pelvis before administration of an epidural.